Leibfried, in U.S. Pat. Nos. 4,900,779, and 4,902,731, and U.S. Patent Application Ser. Nos. 07/419,429 (filed Oct. 10, 1989, now U.S. Pat. No. 5,013,809) and Ser. No. 07/419,430 filed Oct. 10, 1989, now allowed), and Bard and Burnier, in U.S. Patient Application Ser. No. 07/422,214 (filed Oct. 16, 1989, now U.S. Pat. No. 5,008,360 ) describe crosslinked organosilicon polymers and crosslinkable organosilicon prepolymers comprised of alternating polycyclic hydrocarbon residues and cyclic polysiloxanes or siloxysilane residues linked through carbon to silicon bonds. Cowan, in U.S. Pat. No. 4,877,820, discloses crosslinked or crosslinkable linear poly(organohydrosiloxane) polymers having at least 30% of their.tbd.SiH groups reacted with hydrocarbon residues derived from polycyclic polyenes.
These polymers and prepolymers are often stored over long periods of time. In addition, the prepolymers are often cured at high temperatures and the polymers are often exposed to high temperatures during use. They contain residual .dbd.Si--H groups and carbon-carbon double bonds which cause them to be thermally unstable, particularly over long periods of time and especially at elevated temperatures. The instant inventor has discovered that the crosslinked organosilicon polymers and prepolymers can be stabilized using the combination of at least one compound having a hindered phenol substituent and at least one compound having a substituent selected from hindered amines and aromatic amines.